Colorectal cancer is the second deadliest malignancy in the United States. Mutation of the adenomatous polyposis coil (Apc) tumor suppressor gene initiates most colorectal carcinomas. However, it is not known how Apc mutation predisposes a cell to polyp development and colorectal carcinogenesis. Although APC is found at cell-cell junctions, binding to microtubules, and in the nuclei, little is known about nuclear APC function. We discovered increased cytoplasmic APC as human colon tissue progressed from normal, to polyp, to tumor. In cultured cells, APC localization responded to cell proliferation and phosphorylation. Furthermore, using this model we found that nuclear APC regulated the activity of the oncoprotein beta-catenin. We hypothesize changes in APC localization, initiated by mutation of the APC nuclear localization signals, will result in concomitant alterations in beta-catenin regulation, proliferation and differentiation at the cellular level, and polyp formation at the tissue level. We will inactivate APC's nuclear localization signals in mouse embryo-derived stem (ES) cells and whole animals to study nuclear APC function under physiological conditions. We will use these two innovative model systems to test directly if nuclear APC is involved in beta-catenin regulation (Aim 1), cellular proliferation (Aim 2), and differentiation (Aim 3). We will perform pathologic examinations on mice lacking nuclear APC to test if nuclear APC functions in tumor suppression (Aim 4). Greater knowledge of APC function in normal cells will improve our understanding of APC's role in tumorigenesis and ultimately illuminate new points for therapeutic intervention.